With the advancement of ubiquitous network society, the demand for mobile devices such as cellular phones, notebook personal computers, and digital still cameras has been remarkably increased. As the power source for such mobile devices, fuel cells, which need no recharging and will operate as long as fuel is supplied, are expected to be put into practical use as early as possible.
Among such fuel cells, direct oxidation fuel cells, which generate electric power by directly supplying an organic fuel such as methanol or dimethyl ether to an anode for oxidation without reforming the fuel into hydrogen, have been attracting attention, for which active research and development have been performed. The reasons for this include that organic fuels have a high theoretical energy density and are easy to store, and that organic fuels can simplify the fuel cell system.
Direct oxidation fuel cells include a unit cell comprising a membrane-electrode assembly (hereinafter referred to as an “MEA”) sandwiched between separators. The MEA generally includes a solid polymer electrolyte membrane and an anode and a cathode sandwiching the electrolyte membrane, the anode and the cathode each including a catalyst layer and a diffusion layer. Such direct oxidation fuel cells generate power by supplying fuel and water to the anode and supplying oxidant such as oxygen to the cathode.
For example, the electrode reactions of a direct methanol fuel cell (hereinafter referred to as a “DMFC”), which uses methanol as the fuel, are as follows.
Anode: CH3OH+H2O→CO2+6H++6e−
Cathode: 3/2O2+6H++6e−→3H2O
Specifically, at the anode, methanol reacts with water to produce carbon dioxide, protons, and electrons. The protons produced at the anode migrate through the electrolyte membrane to reach the cathode, and the electrons migrate through an external circuit to reach the cathode. At the cathode, the protons and the electrons having migrated through the external circuit combine with oxygen to produce water.
However, there have been several problems in putting DMFCs into practical use.
One of them is a problem regarding the durability. In the interior of the cathode catalyst layer and/or the interface between the cathode catalyst layer and the cathode diffusion layer, with the passage of power generation time, water produced by reaction and/or water transferred from the anode will accumulate in the form of liquid. The accumulated water causes the diffusibility of an oxidant in the cathode to deteriorate and thus the cathodic concentration overvoltage to increase. This is considered to be a primary cause of the initial deterioration of the power generation performance of DMFCs.
This initial deterioration is strongly influenced by methanol crossover (hereinafter referred to as “MCO”). MCO is the phenomenon of permeation of unreacted methanol through the electrolyte membrane to reach the cathode. In other words, in the cathode catalyst layer, oxidation reaction of crossover methanol occurs simultaneously with a reaction that normally occurs at the cathode, namely, reduction reaction of oxygen. For this reason, particularly when high-concentration methanol is used as the fuel, the amount of MCO increases with the passage of power generation time, causing the cathodic activation overvoltage to increase significantly. Moreover, carbon dioxide produced by the reaction causes the diffusibility of the oxidant to further deteriorate, and thus the power generation performance significantly deteriorates.
The initial deterioration as described above tends to occur in the cathode-side power generation region facing the upstream portion of the fuel flow channel where the amount of MCO is large. This initial deterioration becomes notable with the decrease of the three-phase interfaces where the catalyst phase, the electrolyte phase, and the oxygen phase coexist, the three-phase interfaces serving as electrode reaction sites.
In order to address these problems, there has been proposed a method of increasing the catalyst content in a DMFC relative to that in a solid polymer electrolyte fuel cell (PEFC), thereby to increase the surface area of the catalyst (i.e., catalyst reaction sites) per unit area of the catalyst layer. However, an increase in the catalyst content leads to an increase in the thickness of the catalyst layer itself, so that it becomes difficult for the oxidant to reach the reaction sites in the interior of the catalyst layer. As a result, contrary to the expectation, the power generation performance deteriorates.
Hence, as a solution to the above-discussed problems, many proposals have been made to improve the structure of the cathode catalyst layer itself. For example, Japanese Laid-Open Patent Publications Nos. 2005-353541 and 2006-107877 disclose providing the cathode catalyst layer with a plurality of through pores or vertical apertures. Techniques disclosed in these documents intend, by way of providing a plurality of through pores or vertical apertures, to allow the oxidant to be smoothly supplied into a deep portion of the catalyst layer as well as to allow water to be smoothly drained from a deep portion of the catalyst layer even when the thickness of the cathode catalyst layer is increased.
Japanese Laid-Open Patent Publication No. 2006-134886 discloses providing an electrode substrate carrying a catalyst layer with a hydrophilic region and a hydrophobic region that are separated from each other. The technique disclosed in this document intends, by way of configuring the catalyst layer as described above, to facilitate the draining of water from the cathode, thereby to prevent clogging of pores of the membrane with the water.
Japanese Laid-Open Patent Publication No. 2005-183368 discloses setting the thicknesses of the anode catalyst layer and the cathode catalyst layer to 20 μm or more, and providing at least one of the catalyst layers with pores having a pore size of 0.3 to 2.0 μm such that the volume of these pores is equal to or greater than 4% of the total pore volume. The technique disclosed in this document intends, by way of configuring the catalyst layer as described above, to allow liquid fuel and oxidant to be smoothly supplied to the reaction sites in the interior of each electrode without sacrificing the electron conductivity and the proton conductivity.
However, even with the use of the configurations of the conventional techniques as described above, in the cathode-side power generation region facing the upstream portion of the fuel flow channel where the amount of MCO is large, it is impossible to ensure sufficient amount of three-phase interfaces serving as electrode reaction sites. Moreover, in the cathode-side power generation region facing the midstream and downstream portions of the fuel flow channel where the amount of MCO is reduced, it is impossible to maintain the smooth supply of oxidant to the interior of the catalyst layer and the smooth draining of water from a deep portion of the catalyst layer. Therefore, by using the above-described conventional techniques, it is difficult to obtain a catalyst layer that can reduce the cathodic overvoltage.
Specifically, in the case of the techniques represented by Japanese Laid-Open Patent Publications Nos. 2005-353541 and 2006-107877, since a plurality of through pores or vertical apertures are provided, in other words, since large defects are present in the cathode-side power generation region facing the upstream portion of the fuel flow channel where the amount of MCO is large, an appropriate amount of the three-phase interfaces serving as electrode reaction sites cannot be provided and thus the cathodic overvoltage in this region increases. In the cathode-side power generation region facing the midstream and downstream portions of the fuel flow channel where the amount of MCO is reduced, the oxidant readily reaches the three-phase interfaces serving as electrode reaction sites through the plurality of through pores or vertical apertures provided in the interior of the catalyst layer. As such, in the initial stage of power generation when there is not much condensed water accumulated at the cathode, the power generation performance is comparatively good. However, with the passage of power generation time, condensed water is increasingly accumulated in the interior of the through pores or vertical apertures, making it difficult to reliably supply the oxidant into a deep portion of the cathode catalyst layer. Therefore, it is predicted that the power generation performance will drop abruptly.
In the technique disclosed in Japanese Laid-Open Patent Publication No. 2006-134886, the electrode structure is not designed with taking into account the influence of the MCO amount distribution along the flowing direction of the fuel. Because of this, the cathode-side power generation region facing the upstream portion of the fuel flow channel becomes deficient in the three-phase interfaces serving as electrode reaction sites, and thus the cathodic overvoltage will increase.
In the technique disclosed in Japanese Laid-Open Patent Publication No. 2005-183368, the lower limit of the thickness of the catalyst layer, and the size and volume of the pores are merely defined. Therefore, it is difficult to say that an optimum pore structure is realized throughout the entire catalyst layer, the optimum pore structure having all of the followings: the diffusibility of fuel or oxidant, the ability of draining carbon dioxide or water being a reaction product, the electron conductivity, and the proton conductivity.
The invention intends to solve the above-described conventional problems and to provide a direct oxidation fuel cell excellent in power generation performance and durability.